Aggie, Ingeniero November 2011

Engineering research explores new locations for satellites

Space is increasingly becoming more littered with debris — everything from spent rocket stages and defunct satellites to erosion, explosion and collision fragments, all of which pose the risk of colliding with expensive military and scientific satellites. This is but one of several reasons that Eric Butcher, New Mexico State University aerospace engineer, is conducting research that may lead to placement of important space assets further from Earth than they are currently located.

“The risk of collision with space debris is one reason to move satellites higher up, but there are lots of reasons,” said Butcher, who received funding from the U.S. Department of Defense Air Force Office of Scientific Research for his research.

Locating military satellites used for surveillance and communications further from Earth will also make them more secure, rendering them less susceptible to interference and tracking by unfriendly entities. It may also increase global coverage, giving them a wider range of observation, both of the Earth and other satellites. Military satellites traditionally do not venture beyond the altitude of geosynchronous orbits (orbits that repeat regularly above points on the Earth over time) — an altitude of 35,786 kilometers above the Earth.

Butcher’s research centers on special locations in the Earth-Moon system (libration or Lagrange points) where, due to the balance of gravity, a spacecraft may remain motionless in the rotating frame of two bodies such as the Earth and the Moon. While there are five such locations (L1-L5), the research will mostly focus on two of them (L1 and L3). In addition, although libration points were discovered in the mid-1700s, various orbits (called “halo orbits”) near L1 and L2 have been discovered in the past 50 years that would enable good observability characteristics with low stationkeeping costs.

While several scientific spacecraft have ventured to the Sun-Earth libration points, the NASA ARTEMIS mission in 2010 was the first to reach the Earth-Moon points L1 (located between the Earth and Moon) and L2 (located on the far side of the Moon from Earth), each about 61,000 km (38,000 miles) above the lunar surface. The ARTEMIS mission involved two sister spacecraft, which were actually two of the five spacecraft used in the previous THEMIS mission to collect new science data in the Sun-Earth-Moon environment. The two spacecraft were redeployed to the Earth-Moon L1 and L2 locations using low-cost transfers that utilize special pathways called invariant manifolds, which are properties of the nonlinear dynamics of the restricted three body problem, and hence require much less fuel than do traditional lunar transfers. Navigation and stationkeeping data from the NASA ARTEMIS mission will be used in this project.

While halo orbits around L1 and L2 and low-cost transfers to them have received much recent attention by researchers, part of Butcher’s research will focus on these features of the relatively less studied location L3, as well as “exterior weak stability boundary” transfers which involve the spacecraft flying far outside the Earth-Moon system to the vicinity of the Sun-Earth L1 location before returning back, again for less fuel than a direct transfer. Special “tadpole” and “horseshoe” orbits that weave back and forth at approximately the moon’s distance among the L3, L4, and L5 Lagrange points will also be investigated for their coverage characteristics and applicability to military missions.

“It translates to dollars—the less fuel used the less costly the mission,” said Butcher. “Or, a spacecraft carrying less fuel could have a larger payload. The disadvantage of these low-cost transfers is a much longer flight time, but this may not be as important for non-human missions.”

Butcher and others at NMSU will work on this project with researchers in the Aerospace Engineering Sciences Department at the University of Colorado at Boulder and at the Air Force Research Laboratory in Albuquerque. They will investigate the use of libration point orbits for military satellites, study the coverage characteristics from these locations, find new low-cost transfers, and also study autonomous navigation and stationkeeping strategies for these missions.

This three-year project is the first funded grant in aerospace engineering at NMSU that concerns topics in orbital mechanics and astrodynamics and will benefit students in the aerospace engineering program. A portion of the budget is dedicated to building a new visualization facility in Jett Hall for research and instruction in orbital mechanics and spacecraft trajectories. Additionally, Butcher is teaching a new graduate-level aerospace engineering course in astrodynamics which deals with the topics in this project and will prepare students to participate in the research. He also teaches an undergraduate course in orbital mechanics.

While Butcher’s project is aimed specifically toward military uses, the information will be equally applicable for scientific missions.

“Future scientific robotic and manned missions might take us to the far side of the moon where direct line-of-sight communication with the Earth is impossible, so we could also have a communications link satellite in a large halo orbit around L2 on the far side of the moon, from which both the lunar far side and the Earth would be visible simultaneously and continuously. There has been lots of discussion about this in the research community,” said Butcher.

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